Pass filters from buck test to gtest

[nataliejameson]

Right now there isn't a way to filter tests in the gtest runner. We can accomplish this by just passing an argument from the user to --gtest_filter. The main thing here will be that we probably can't reuse --filter, as its test format is not the same as gtest. Once we fix the hard part of naming the flag, it should be simple enough to pass through to GtestTest (and probably boost test too probably).